Summary Background: Peristomal skin problems are common and are treated by a variety of health professionals. Clear and consistent communication among these professionals is therefore particularly important. The Ostomy Skin Tool (OST) is a new assessment instrument for the extent and severity of peristomal skin conditions. Formal tests of reliability and validity are necessary for its use in clinical practice, research, and education. Objectives To estimate inter- and intra nurse assessment variability of the OST and validity by comparison to a 'gold standard' (GS) defined by an expert panel. Methods Thirty photographs of peristomal skin were presented twice to 20 ostomy care nurses - 10 from Denmark (DK) and 10 from Spain (ES) - to determine intra- and inter nurse assessment variability. The same photographs were presented to an international group of experts (dermatologist and ostomy care nurses), to establish a GS for comparison and validation of the results. Results A high intra-nurse assessment agreement, κ = 0·84, was found with no differences in the intra-nurse assessments from the two groups of nurses (DK and ES). The inter-nurse assessment agreement was 'moderate to good', κ = 0·54, with the agreement between the experts higher, κ = 0·70. A high correlation between the scores from the nurses and the GS were seen in the lower part of the two scales [Discoloration, Erosion, Tissue overgrowth (DET) score < 7)]. Conclusion The study supported the validity of the OST. It is suggested that a categorical scale can be used to illustrate the severity of the DET scores.